On a dark, rainy night Cass takes a shortcut home down a treacherous road after promising her husband that she wouldn't go that way. When she sees a car pulled off the side of the road, she momentarily stops, wondering if the person needs assistance. She can just make out the woman inside the car, but the woman doesn't try to signal her or otherwise indicate she needs any help, so with the rain pouring down, Cass drives on home. The next day Cass learns that the woman was found murdered in her car. And to make matters worse, it was a woman she was acquainted with. Now Cass is racked with guilt that she didn't do more to try to help the woman. Add this to the fact that she seems to be forgetting things with increasing regularity and Cass fears she may be losing her mind. While this may not be an entirely original story idea, it does work well here. There's no big surprise as to who the bad guys are; the beauty is in how they manipulate Cass.